In copying machines and laser beam printers, a method has been widely employed in which copy paper on which a toner image is formed is passed between a heating fixing roller provided with a heating source inside and a pressure roller, whereby unfixed toner is melted by heating and the toner image is fixed onto the paper. As the heating fixing roller, a roller which includes a base member composed of a metal tube or a heat-resistant plastic tube, such as a polyimide tube, and a fluororesin layer (release layer) for preventing adhesion of molten toner, the fluororesin layer being provided on an outer circumferential surface of the base member, is widely used.
In recent years, with the increase in copying speed, scattering of a toner image on copy paper due to the occurrence of static electricity, and occurrence of electrostatic offsets have become more of a problem. The electrostatic offsets are divided into two types: a total surface offset and a peeling offset. The total surface offset easily occurs when the surface resistance of the release layer is low, while the peeling offset easily occurs when the surface resistance is high. In order to suppress both types of offsets, the acceptable range of surface resistance is required to be very narrow. Therefore, in order to prevent the problem described above, the surface resistance of the fluororesin layer (release layer) is required to be precisely controlled in the heating fixing roller.
In order to decrease the surface resistance of the heating fixing roller, a method has been conceived in which a conductive material, such as carbon black, metal powder, or graphite, is added to the release layer (Patent Document 1). However, when metal powder is used, because of reaction with water in a dispersion (coating material) for forming the fluororesin layer (release layer), oxidation due to high-temperature firing during formation of the fluororesin layer (release layer), or the like, physical properties, such as a releasing property of the fluororesin, are easily degraded, which is a problem. On the other hand, in the case of carbon black or graphite, the dispersion state easily varies and aggregation (percolation) easily occurs. Therefore, it is difficult to stably obtain the intended surface resistance, thus making it difficult to satisfy recent requirements.
As a method of preventing aggregation of carbon black or graphite, use of a semiconductive inorganic substance as an electric potential stabilizer together with carbon black or graphite is known. Patent Document 1 discloses combined use of about 0.3% to 8% by weight of a good conductive substance (conductive material), such as carbon black or graphite, and 5% to 50% by weight of a semiconductive inorganic substance, such as titanium oxide, iron oxide, aluminum hydroxide, talc, barium titanate, antimony oxide, silica, or calcium carbonate, and by the combined use, aggregation of carbon black, graphite, or the like can be prevented to a certain degree (Patent Document 1, paragraph 0024).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-125404